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Published on August 17, 2022
Phentermine is a class IV stimulant drug that should only be available with a doctor’s prescription. It has at least a low potential for drug abuse and when used incorrectly may lead to serious side effects. These unwanted effects are more likely to occur in people with specific medical histories or who take certain medications together. (1)
Therefore, before prescribing phentermine, your doctor needs to know your complete medical history, current health status, and list of medications and supplements. (1)
Pre Existing Conditions
Please inform the prescribing doctor of your complete medical history during your initial consultation. You should tell your doctor if you have ever had any of the following conditions, as they carry additional phentermine warnings and may prevent you from taking this medication. (1)
Patients with a known history should NOT take phentermine.
Cardiovascular disease includes coronary artery disease, stroke, irregular heartbeat (arrhythmia), congestive heart failure, and uncontrolled hypertension. Hyperthyroidism, glaucoma, anxiety, insomnia, mania, and all in agitated mental states. (1)
Also, pregnant and nursing women should not take this medication. Phentermine is considered in the highest risk category (X) by the FDA for use during pregnancy. (1)
Interactions Between Medications
As a potent central nervous system stimulant, phentermine causes significant physiological changes, some of which may be problematic when combined with other drugs. Thus, many of the phentermine warnings concern possible drug interactions. (1)
You must talk to your doctor and pharmacist about their possible drug interactions if you’re taking prescription, over-the-counter, or dietary supplements. (1)
1. Psychiatric Drugs
Drugs used to treat mental health issues like anxiety and depression often have harmful interactions with phentermine. Antidepressants like monoamine oxidase inhibitors (MAOIs), tricyclic antidepressants (TCAs), selective serotonin reuptake inhibitors (SSRIs), and serotonin and norepinephrine reuptake inhibitors (SNRIs), as well as phenothiazine antipsychotics, should all be disclosed to your doctor. (2)
Monoamine Oxidase Inhibitors
Antidepressants in the monoamine oxidase inhibitor (MAOI) class effectively treat depression and Parkinson’s disease. Due to its high potential for food and drug interactions, this early type of antidepressant has fallen out of favor, but it is still used in cases where other medications have failed. (2)
A hypertensive crisis is more likely to occur if phentermine is taken alongside or within 14 days of taking an MAOI. Stroke is just one of the many potentially fatal complications that can arise from a hypertensive crisis, a sudden and severe rise in blood pressure. (2)
Older medications called tricyclic antidepressants (TCAs) are effective against depression, obsessive-compulsive disorder, and manic-depressive illness. TCAs, like MAOIs, have largely fallen out of favor because they have more severe side effects and overdoses than newer antidepressants. (2)
Avoid taking TCAs with phentermine because they increase the body’s response to the stimulant by inhibiting norepinephrine reuptake. Furthermore, the pressor response is amplified by this combination, leading to potentially life-threatening elevations in blood pressure. Headaches, trembling, chest pain, and an abnormal heart rate or palpitations are all possible side effects. (2)
Inhibitors of Selective Serotonin Reuptake
Newer antidepressant medications, such as selective serotonin reuptake inhibitors (SSRIs) and serotonin/norepinephrine reuptake inhibitors (SNRIs), raise serotonin (and norepinephrine, in the case of SNRIs) levels in the brain to alleviate depression. Because of their low risk of adverse effects, SSRIs have become the most widely used class of antidepressants. (2)
You shouldn’t combine phentermine and SSRIs or SNRIs unless your doctor tells you to. It is possible that serotonin syndrome, a potentially fatal medical condition, could result from taking these two drugs together.
Symptoms of serotonin syndrome include irritability, hallucinations, a racing heart, a low core temperature, profuse perspiring or shivering, wildly fluctuating blood pressure, tremors, abdominal cramps, nausea, vomiting, and diarrhea (among other symptoms). (2)
Recent randomized clinical trials have shown that some patients can take SSRIs/SNRIs (but not MAOIs or TCAs) while taking low doses of phentermine or phentermine/topiramate (Qsymia). Patients were excluded from the study if they had suicidal thoughts, had experienced significant depression more than once, or had a stable dose for at least three months before starting phentermine. (2)
When treating severe mental and emotional health disorders like schizophrenia and agitation, a class of antipsychotic medication known as phenothiazines (PTZ) is often the first line of defense. In addition to treating severe hiccups, severe nausea and vomiting, moderate to severe pain, and porphyria, some formulations of this drug are used to manage these conditions. (2)
Suppose you’re trying to lose weight with phentermine. In that case, you shouldn’t take any phenothiazines simultaneously because it can mess with the drug’s appetite-suppressing effects and could make your psychosis worse (e.g., hallucinations or delusions). (2)
2. Alternative Weight Loss Drugs Or Dietary Supplements
If you have tried ANY other weight loss medications or supplements in the past year, including (but not limited to):
- Diethylpropion and Benzphetamine (Didrex) (Tenuate)
- Lorcaserin (Contrave) and Liraglutide (Saxenda, Victoza) (Belviq)
- Combined long-acting bupropion and naltrexone (Contrave)
- Phentermine (Adipex) and topiramate (Ambien) extended-release, orlistat (Xenical), and phendimetrazine (Bontril) (Qsymia)
- Sibutramine (Meridia)
- Herbal supplements and other OTC aids for weight loss include:
- Ephedrine and Orlistat (Alli) (ephedra, ma-huang)
- Phenylpropanolamine (Acutrim, Dexatrim, Mega-Trim)
3. Various Stimulants
Because of its potent sympathomimetic (nerve-stimulating) effects, phentermine should never be taken alongside other stimulants. (2)
Hypertensive crisis and cardiac arrhythmia are just two of the cardiovascular and central nervous system complications that can result from taking multiple stimulants at once. Combining phentermine with other drugs that contain psychostimulants is risky because of the prevalence of these substances in both legal and illegal pharmaceuticals. (2)
If you’re taking phentermine, don’t take any of these other stimulant drugs at the same time:
- Acebutolol (Sectral)
- Amphetamine (Adzenys XR-ODT, Dyanavel XR, Evekeo)
- Dextroamphetamine (Dexedrine)
- Adderall plus dextroamphetamine (Adderall)
- Epinephrine/Norepinephrine (EpiPen, Adrenalin, Levophed)
- Lisdexamfetamine (Vyvanse)
- Methylphenidate (Ritalin, Concerta)
- Midodrine (ProAmatine)
- Phenylephrine (Sudafed)
- Afrinol, Sudafed, and Sinutab contain pseudoephedrine
Or illegal stimulants found in the “street drug” market, such as:
- Amphetamine (speed)
- Cocaine (coke, snow, blow, crack)
- Methamphetamine (meth, ice)
- MMDA (molly, ecstasy, E)
4. Sleep-Inducing Medications
Stimulant effects of phentermine include a rise in alertness and a decrease in food intake. The risk of adverse side effects, such as high blood pressure, is increased when this sympathomimetic is combined with sedatives. (2)
Please inform your doctor if you are taking any of the following sleep aids:
- Drugs like codeine and hydrocodone, which are opioids, are used to treat pain and coughing.
- Anxiety and sleep aids like Xanax, Lorazepam (Ativan), and Zolpidem (Ambien)
- Calming drugs for muscles, like carisoprodol (Soma) or cyclobenzaprine (Flexeril)
- Cetirizine (Zyrtec), diphenhydramine, and other antihistamines (Benadryl)
It is essential to check the labels to avoid taking any new sleep aids. Before adding any new medication or supplement to your phentermine regimen, talk to your pharmacist. (2)
5. Medications for High Blood Pressure
Increased blood pressure is a possible side effect of phentermine. A reduction in blood pressure medication use may occur over time if a person loses enough weight. (2)
Phentermine, a weight loss medication, can reduce the effectiveness of beta blockers, a type of blood pressure medicine. Furthermore, patients are at risk for unopposed alpha-adrenergic stimulation, leading to elevated blood pressure, slowed heart rate, and heart block when these substances are used together. (2)
6. Diabetes Medication
The ability of the body to regulate glucose levels may be significantly altered by phentermine. Some people with diabetes may not be good candidates for this treatment.
The stimulant raises “fight or flight” hormone levels in the blood, increasing liver glucose production and possibly reducing cellular glucose uptake. Lifestyle changes aimed at weight loss, such as eating less or exercising more, can also help lower blood sugar levels. (2)
Your need for diabetes medication may be affected by both types of variations. (2)
If you have diabetes, tell your doctor about any oral or injectable medications you take, and keep a close eye on your blood sugar levels while taking phentermine.
If you have diabetes, discuss your current blood sugar control and the safety of phentermine for weight loss with your prescribing doctor before starting treatment.
Phentermine Lifestyle Cautions
There is a long list of phentermine contraindications related to potential drug interactions, but some behaviors should be avoided while taking phentermine weight loss pills. (2)
Alcohol and Phentermine
Do not combine phentermine with any alcoholic beverages. (2)
Phentermine stimulates the central nervous system, whereas alcohol has a sedative effect.
Addictive effects on the heart and brain, such as:
- An elevated heart rate
- A rise in blood pressure
- Pain in the chest
- Problems focusing
Also, remember that marijuana (cannabis) can make you feel more lightheaded and sleepy. If you use marijuana, you should discuss it with your prescribing physician. (2)
You should avoid or cut back on caffeine if you’re trying to lose weight while taking phentermine. To stimulate the central nervous system, as phentermine and caffeine do. The additive effect of the two substances may increase the potential for adverse reactions if used together.
Caffeine can be found in many foods and drinks, including coffee, chocolates, energy drinks, sodas, and tea. (2)
Powering and Controlling Massive Machinery
Before knowing how phentermine affects you, avoid driving or use caution when operating heavy machinery. As a side effect, this medication may make you feel less tired than you are. Before driving or operating heavy machinery, wait until you know how this medication affects you. (2)
Medical professionals (doctors, dentists, nurses, etc.) should be aware that you are taking phentermine if you are hospitalized or undergoing surgery. The myocardium becomes more sensitive to phentermine’s stimulating effects after being exposed to inhaled anesthetics, which are used for in- and outpatient procedures.
Combining these two medications raises the risk of arrhythmia because of the sympathomimetic’s tendency to increase heart rate and blood pressure. This is a highly problematic additional step. (2)